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| Book/Dissertation / PhD Thesis | FZJ-2020-01527 |
2019
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-430-0
Please use a persistent id in citations: http://hdl.handle.net/2128/24595 urn:nbn:de:0001-2020060532
Abstract: Renewable energy sources such as wind and solar energy shall cover a large part of theelectricity demand in the future. However, the natural fluctuation of these energy sources leadsto a fluctuating electricity production, which is not always in accordance with demand. Thestorage of surplus electricity and the delayed reconversion in case of electricity demandrepresents a solution to counteracting this issue. Due to the large amount of energy, a chemicalstorage is particularly suitable for this purpose. To realize this solution, conversion of electricalinto chemical energy with solid oxide cells by steam electrolysis can takes place first. Theproduced hydrogen is temporarily stored and will be reconverted at electricity demand with thesame solid oxide cells by fuel cell operation. This offers an economic advantage over systemswith two solid oxide cell stacks. Due to the pure hydrogen/steam operation, this reversible solidoxide cell system (rSOC-system) is environmentally friendly and thereby differs from the knownrSOC-systems which are using carbonaceous energy sources.The main objective of this work is the research of a technically and economically efficienthydrogen-powered rSOC-system. First, the developments of the system design and of theoperating strategies, which should ensure a safe and fast operating point change, have priority.In order to develop a highly efficient rSOC-system, the system components were investigatedexperimentally. Based on this, dynamic component models were created, which were validatedwith the help of the examination results. The ensuing system models are basing on theinterconnection of the validated component models and allow investigations at system level.The final economic analysis represents the economic viability of the investigatedrSOC-systems. This analysis takes different quantities, system sizes and system designs intoaccount. The influence of varying electricity generation costs and operating hours is shown ina sensitivity analysis.First, an rSOC basic system was developed that only contains necessary components foroperation. This system achieved an efficiency of 43.4% and is the starting point for theinvestigation of efficiency enhancing-measures. On the one hand, the efficiency could beincreased to 45.5% via an internal heat recovery in electrolysis operation and, on the otherhand, the parasitic system consumption could be significantly reduced with the integration ofan off-gas recirculation. In addition, a condenser is included in the recirculation which mainlyenables the recirculation of hydrogen and leads to an efficiency increase to 49.9%. Theoperating strategies developed for this final rSOC-system enables, inter alia, a system startupin two hours. The load change from full load to 50% partial load takes ten minutes in fuel celloperation and three minutes in electrolysis operation. Switching between these two operatingmodes is possible in less than five minutes. For the solid oxide cells critical operating areaswere analyzed and avoided. The economic analysis showed that a 5 kW rSOC-system can beeconomical from 100 units upwards in comparison to mains supply. This system pays for itselfafter only seven years, resulting in electricity generation costs of € 0.232 / kWh (100 units) upto € 0.148 / kWh (100,000 units). rSOC-systems with higher power output (≥ 50 kW) pay forthemselves later and at higher quantities because the competing electricity price by mainssupply decreases with increasing purchase quantities.
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